The present invention relates generally to stream selectors for process analyzers.
Stream selectors for process analyzers control the flow of fluid into the analyzer. The selector selects a single sample stream from multiple flow streams to pass on for analysis. This reduces the cost of analyzing multiple gas and liquid process streams in, e.g., a manufacturing or laboratory facility, as each analyzer is relatively expensive. The stream selector includes a series of valves which are typically electrically controlled. It is conventional practice to use a common outlet header connected to each valve to route the selected sample stream to the analyzer. It is important that only one sample is routed through the header to the analyzer at one time.
Important factors in analyzing process streams are i) cross-contamination of samples; ii) the size of the analyzer and associated components; and iii) the ease of installation, maintenance and repair of the analyzer. Cross-contamination of samples can be caused by leaking valves and/or dead volume (e.g., irregular passageways, large internal volumes, etc.) allowing contamination from previous samples. To overcome this problem, longer sample purge times and/or stream analysis have been used. However, this increases the time and cost associated with process analyzing, as well as requires the disposal of the greater purge volume. The size of the analyzer and related components is also an issue as large and bulky analyzers take up valuable panel space. The ease of installation, maintenance and repair of the analyzers is also an important consideration as there is a continuing demand to reduce the cost of the analyzers, and hence minimize the cost of the entire process system.
Certain stream selectors have been developed in an attempt to address some of these issues. For example, one stream selector is known which has a double block and bleed structure, where a set of three O-rings are carried on the head of a poppet valve. The poppet valve is normally biased into a closed (non-actuated) position by a compression spring, and can be moved into an open (actuated) position by pressurized gas or other means. The O-rings are designed to seal against opposed flat surfaces in the selector body to control the flow of fluid through the body. A first and second of the O-ring seals are located in grooves formed in one surface of the poppet valve head to seal against the inlet and outlet passages, respectively, when the selector is not actuated; while a third of the O-rings is loosely located around the poppet stem against the opposing surface of the poppet valve head and is designed to seal against a vent passage when the selector is actuated. An intermediate position is also provided, where the poppet valve head is in a position where all passages are open to completely purge any fluid in the selector system.
The double block and bleed structure is located in a valve module, and a series of such modules can be arranged adjacent one another to form the stream selector. A common outlet passageway (loop) is typically required between each valve module and the analyzer. The common outlet passageway allows purging of the passageway between different samples.
It is believed the O-ring seals in the selector described above can dry and crack, and/or swell and dislodge during repeated cycling, particularly in liquid applications. This can allow leak paths to occur between the inlet and outlet passages, and also through the vent passage. The selector also does not fully minimize the volume between the valve modules and the analyzer, as it requires an outlet loop. Providing an outlet loop increases the overall size of the selector, as well as adds additional cost to the system. The selector described above also requires the adjacent valve modules to be firmly pressed together and connected with long bolts extending horizontally through the modules. The bolts must be removed and all the modules disturbed if one of the modules is to be replaced. Since this can be quite an involved operation, the selector typically must be taken to a repair shop remote from the application. Process connections are also required for each module, so that the process connections for typically all modules must be disconnected when the one module is to be replaced. Leak paths can be introduced into the selector during all these disconnections and reconnections. Further, the stream selector of this design tends to be complex and include small C-clips and hidden roll pins, which makes assembly, maintenance, and repair, time-consuming and expensive.
It is also known to provide ball valves in stream switching, however, such ball valves are not known for their robust design during repeated cycling, and typically must be inspected and repaired or replaced at regular intervals. Selectors using such ball valves also tend to be quite large and require considerable space.
In light of the above, it is believed there is a demand for an improved stream selector for a process analyzer which prevents cross-contamination of samples, has a relatively small size, and has a simple valve structure which facilitates installation, maintenance and repair of the selector to minimize costs.
The present invention provides a novel and unique stream selector for a process analyzer which prevents contamination of one sample from previous samples. The stream selector does not require a common outlet loop to connect the valves to the process analyzer, and has robust, long-lasting seals to prevent leakage. The stream selector also has modules of a compact size, which are relatively simple in construction and are individually and separately removeable. This reduces installation, maintenance and repair of the selector and minimizes costs of the entire system.
According to the present invention, the stream selector includes a series of base plates, and a series of valve modules which are individually removeably attached to the base plates. The base plates are connectable to each other via interengaging bolts, and include all the process connections for the fluid lines. This facilitates the assembly, maintenance and repair of the stream selector, as well as reduces the complexity of the valve modules.
Each valve module for the selector includes a module body with a pair of valve cavities. A valve assembly is received in each valve cavity. Each valve assembly includes a valve body enclosing a moveable piston, a valve bonnet, and a poppet valve. The valve assemblies are preferably operated simultaneously through a common actuation passage.
The poppet valve in each assembly extends through the bonnet and is connected to the piston, and moves in conjunction therewith. The poppet valve has a valve head with a double-seated seal. The double seated seal has on one side an annular plug seal received around the stem of the poppet valve for sealing to a first valve seat to control the flow of fluid through the vent passage in the valve body; and on the other side includes a solid cylindrical plug seal which seals to a second valve seat to control the flow of fluid between the inlet and outlet passages in the valve body. The first and second plug seals are preferably received in respective seal holders on the opposite surfaces of the head, and provide long-lasting, fluid-tight sealing over repeated cycling.
The outlet passage from one valve assembly in the module is routed to the inlet passage of the adjacent valve assembly in the module to create a double-block and bleed configuration. The downstream valve assembly in the module is then connected to the process analyzer. This configuration prevents downstream contamination of samples, and purges the valve module during each stream selection. No outlet loop is necessary to purge the modules.
If more than one valve module is used for the stream selector, the outlet of the downstream valve assembly in the first module is routed through the vent passage of the downstream valve assembly in the next module to purge residual fluid in the second module when the first module is actuated. This configuration is replicated for all the modules in the stream selector, and completely purges the selector of previous samples to prevent cross-contamination.
A cover encloses each valve assembly in the valve cavity of the module. The cover is attached by a series of threaded bolts to the module body. At least some of the bolts pass through the module body and fix the module body to the base plate. Loosening the bolts allows individual modules to be removed from the base plate without removing adjacent modules. This also allows the valve assemblies to be easily removed from each module.
As indicated above, the stream selector does not require a common outlet header (loop) to connect to the process analyzer. Rather, the outlet from the last module in the string of modules is connected directly to the analyzer. The previous process streams are fully vented before the introduction of a new process stream into the last module. The stream selector thereby prevents contamination of one sample with residual fluid from a previous sample, and has little dead volume. Still further, the stream selector has a compact size, and is relatively simple in construction. This also reduces installation, maintenance and repair costs. The seal plugs of the poppet valve are robust, long-lasting components that withstand repeated cycling without leaking. The modules each easily connect into and disconnect from the base plates, which carry all the process connections. This also simplifies installation, maintenance and repair of the stream selector valve.
Further features of the present invention will become apparent to those skilled in the art upon reviewing the following specification and attached drawings.